2. Mangatawa catchment consents for earthworks, storm water ...

NZ Transport Agency and Tauranga City Council
Mangatawa Drain Flood Bypass and TEM Stormwater AEE
through the use of appropriate sediment control devices, as outlined in the Bay of Plenty Regional Council’s
Erosion and Sediment Control Guidelines for Land Disturbing Activities (2001).
Mangatawa Flood Bypass
As with the proposed TEM roadway Stormwater option, a main concern is the potential to cause temporary
negative effects on the downstream receiving environments, through sediment inputs from construction and
maintenance operations. In addition during operation the flood control structures that convey water need to
be able to allow for fish passage. This also applies to the upstream migration of native fish and any instream
works should be carried out, outside of key migration periods.
The temporary negative effects caused by excess sediment loads should be minimised through the use of
appropriate sediment control devices, as outlined in the example erosion and sediment control plan attached
as Appendix 13. This has been informed by the Bay of Plenty Regional Council’s Erosion and Sediment Control
Guidelines for Land Disturbing Activities (2001).
With respect to the freshwater ecology of the Mangatawa Drain, as the proposed activities will result in an
increase in the amount of riparian vegetation present, this will be of benefit to the aquatic habitats within the
drain, providing shading and cover for fish and macro invertebrates, and increase the potential for the drain to
act as a pathway for native fish inhabiting the upper catchment. As the riparian planting will be largely
confined to the lower reaches of the drain it is unlikely to result in a significant change to the water quality of
the existing drain.
Stormwater Treatment Wetland
i. Vegetation
The element that has the potential to have the most significant effect on the ecology of the area, because of
the quality of the existing habitats, is the enlargement of the Stormwater Treatment Wetland and the outlet to
Rangataua Bay.
With respect to the vegetation and flora within the stormwater treatment wetland, the enlargement of this
pond preserves most of the large cabbage trees on this site, then the effect of the overall enlargement on this
component of the vegetation will be no more than minor.
As an enlarged bund will separate the salt-marsh of the SES from the treatment wetland this is likely to
promote the growth of the jointed rush which already dominates this area of the pond. The planting of Carex
in the pond and the forebay has the potential to provide nesting habitat. With respect to the Baumea, shown on
the planting plans in both the stormwater pond and the forebay, this could grow to such a degree as to
eventually fill the pond system, and therefore has the potential to lower the overall capacity of the pond.
Provision needs to be made for several ramps down to water level to allow periodic cleaning of the pond.
With respect to the potential effects of this proposed development and increased discharge of treated
stormwater into Rangataua Bay examination of existing data on the sediment quality and ecology of the bay
(Section 6) indicates that the proposed increased discharge to the Bay will not have a significant adverse effect
of the water, sediment and overall ecology of the Bay.
Status: Final Page 87 April 2009
Project Number: 3932036
NZ1-1552092-Mangatawa Drain Flood Bypass and TEM Stormwater AEE